In electronic time-keeping apparatus, the use of the frequency regulated AC power line signal as a time-base is preferred over the use of a crystal oscillator signal when the former choice is practical, as in nonmobile applications, because crystal controlled oscillators or the like can drift over a period of time thereby introducing time-keeping errors. Likewise, in a television receiver the vertical sync signal previously has been used as a time-base signal for time-keeping apparatus. However, such use of the vertical sync signal also introduces time-keeping errors. These errors result because the time base is lost when the television receiver is turned off, and because the time base is interrupted whenever channels are changed or when an alternate source of video signals, such as a VCR, videodisc player, or a video game is selected. Long term stability is clearly desirable for time-keeping purposes, and the AC power line signal is a convenient source of a stable time-base reference signal.
Although North America has chosen 60 Hz for its power line frequency, a large portion of the world has chosen 50 Hz as a power line frequency. An electronic clock designed to use 60 Hz signals as a time-base will count at 83.33% of its normal rate if provided with 50 Hz signals, resulting in a loss of 10 minutes per hour. On the other hand, an electronic clock designed to use 50 Hz signals as a time-base will count at 120% of its normal rate if provided with 60 Hz signals, resulting in a gain of 12 minutes per hour. In both cases, the clocks are essentially inoperative.
It would be economically undesirable to design an electronic time-keeping apparatus which is unable to use either 50 Hz or 60 Hz signals as a time-base, since to do so would severely limit the marketability of the apparatus. However, if the apparatus is to operate correctly at either 50 Hz or 60 Hz some means must be provided to indicate to its counting circuitry which frequency will be used as the time base. Prior art systems have addressed this problem by providing an extra input terminal which is jumpered or hardwired to a logic reference voltage to act as a "flag" signal. The logic state of this flag signal indicates which of the two frequencies is to be used as the time base. A system of this type is known, for example, from the National Semiconductor MOS Databook, 1980, pages 6-43 to 6-52, AN-196.
As real time clock applications become more complex, the number of available external connection terminals becomes critical, making it undesirable to dedicate a terminal to such a time base "flag" function. The installation of jumper wires during manufacturing is a costly and time-consuming procedure which requires knowledge of the geographic area in which the particular unit being assembled will be sold. The state of the jumper wire flag must be reversed if, for example, a particular unit originally intended for sale in North America is to be shipped instead to a 50 Hz market. An error in the placement of the jumper wire flags at the factory will result in the shipment of essentially inoperative units to the buyer. Likewise, a unit purchased in a 50 Hz market cannot operate without modification when transported to an area using 60 Hz as the power line frequency.